1. Field of the Invention
The present invention relates to hole cutters, and more particularly, to cutters mounted on machine tools to form holes.
2. Description of the Related Art
Hole cutters are mounted on machine tools to form holes in workpieces. Japanese Unexamined Patent Publication No. 1-199710 describes such a hole cutter. As shown in FIG. 7(a), the cutter has a cylindical body with a plurality of flat first cutting teeth 71 provided on the peripheral section of its base end. The cutting teeth 71 are arranged having a predetermined interval between one another. A first blade edge 72 is provided at the distal end of each first tooth 71. The blade edge 72 extends along a direction perpendicular to the axis of the body. The angles .theta. between the blade edge 72 and the left and right sides of the tooth 71 are approximately 90 degrees. Left, right, and middle sections 71a, 71b, and 71c are defined on each tooth 71.
A triangular second cutting tooth 73 is arranged between each pair of adjacent first teeth 71 on the end face of the body. As shown in FIG. 7(b), a second blade edge 74 is provided at the distal end of each second tooth 73. The center of each blade edge 74 projects downward. Thus, the left and right sides of the blade edge 74 extend obliquely toward the center. Left, right, and middle sections 73a, 73b, 73c are defined on each tooth 73. The tooth depth, that is, the depth of the workpiece that may be cut by the tooth, at the left and right sections 73a, 73b of each second tooth 73 is smaller than that of the left and right sections 71a, 71b of each first tooth 71. The tooth depth of the middle section 73c of each second tooth 73 is greater than that of the middle section 71c of each first tooth 71.
To use the hole cutter of the above structure, the blade edges 72, 74 of the associated teeth 71, 73 are abutted against the surface of a workpiece. In this state, the body of the cutter is rotated about its axis at a predetermined circumferential speed and moved toward the workpiece at a predetermined feed rate. This causes the blade edges 72, 74 of the respective teeth 71, 73 to machine the workpiece. Machining of the workpiece causes a groove 75 to be formed in the workpiece, as shown in FIG. 8(a). The left and right portions at the bottom of the groove 75 are machined horizontally by the first teeth 71. The middle portion of the groove 75 is machined into a substantially V-shaped valley that consists of inclined surfaces 75a, 75b.
As shown by the broken line in FIG. 8(a), the first teeth 71 remove flattened strip-like cutting chips 76a, 76b from the bottom left and right portions of the groove 75. As shown by the broken line in FIG. 8(b), the second teeth 73 remove cutting chips 77 having a V-shaped cross-section from the bottom middle portion of the groove 75. Accordingly, rotation of the body results in the first and second teeth 71, 73 machining three separate portions (left, right, and middle) in the bottom of the groove 75. This narrows the width of each cutting chip 76a, 76b, 77 and facilitates the discharging of the chips 76a, 76b, 77. Such width is shown, between sides 77a and 77b of the chip 77 in FIG. 8(a).
When using the above hole cutter, the triangular second teeth 73 remove V-shaped cutting chips 77 from the bottom middle portion of the groove 75. The left and right sides 77a and 77b the chips 77 are each pressed against the bottom of the groove 75 in directions perpendicular to the inclined surfaces 75a, 75b. This causes the right and left sides 77a and 77b respectively of the chips 77 to bend toward each other and entangles the chips 77. As a result, it is difficult to discharge the chips 77 out of the groove 75.
In addition, when the amount of entangled chips 77 becomes large, it becomes further difficult to discharge the chips 77 out of the groove 75.
Furthermore, when machining the left and right portions of the groove 75 with the first teeth 71, frictional force is produced between each first blade edge 72 and the left wall, the right wall, and the bottom of the groove 75. This results in the left and right sections 71a, 71b of each first blade edge 72 being subject to damage.